Add ISSUE_TEMPLATE.md

This is a new Github feature:

  https://github.com/blog/2111-issue-and-pull-request-templates

ISSUE_TEMPLATE.md

@@ -0,0 +1,9 @@
+- Neovim version:
+- Operating system:
+- Terminal emulator:
+
+### Actual behaviour
+
+### Expected behaviour
+
+### Steps to reproduce